1. Field of the Invention
This invention relates generally to a magnetic resonance imaging method, and more particularly to a magnetic resonance imaging method which will be suitable for removing an artifact resulting from the respiratory motion when the image of the abdominal region of a person is formed.
2. Description of the Prior Art
The nuclear magnetic resonance signal S(t, n) (hereinafter referred to as the "NMR signal") obtained from a movable sample is expressed by the following formula as will be described later in further detail: EQU S(t, n)=S.sub.o (t, n)exp(j.DELTA..theta.(n)) (1)
Here, S.sub.o (t, n) is the NMR signal from the sample at rest and .DELTA..theta.(n) is the phase shift of the NMR signal resulting from motion. When the sample oscillates periodically, this .DELTA..theta.(n) becomes a periodic function and results in a motion artifact. The technique of removing the motion artifact is the one that processes this .DELTA..theta.(n) and has been improved since 1984 to the present moment through the following first to fourth stages.
The technique of the first stage detects the respiration and the body motion by use of an air bag, bellows, or the like, and conducts imaging in synchronism with them, as described, for example, in Japanese Patent Laid-Open Nos. 77747/1985, 25542/1986 and 154655/1986. This system keeps the .DELTA..theta.(n) value in the formula (1) constant and removes the motion artifact. However, this system involves the problems that the imaging time is long, and that a specific apparatus must be fitted to the sample or the object to be examined.
Therefore, the technique of the second stage has been proposed which selects the quantity of phase encode in accordance with the detected body motion, collects signal data and later rearranges the data (e.g. "NMR Medical Science", Vol. 6, S.1, p.183 (1986)). This means that .DELTA..theta.(n) in the formula is not the periodic function based on the respiration period but is transformed to a suitable form. Though this technique can reduce the imaging time as the problem of the first stage technique, the problem of fitting of a specific apparatus yet remains unsolved. Since the fitting of the specific apparatus is uncomfortable to the patient, a system not requiring the detector has been desired earnestly.
Recently, the system of the third stage which does not require the detector of the respiration and body motion has been proposed. The system of the the third stage includes the following systems.
3-1) The phase encoding gradient sequence is made non-monotonic, and rearrangement is made after signal data collection to eliminate periodicity of body motion and to remove the artifact. In other words, this system eliminates periodicity of .DELTA..theta.(n) and converts it to noise (e.g. Japanese Patent Laid-Open Nos. 124855/1986 and 155740/1986).
3-2) The system in which the repetition time of the pulse sequence is set to some multiples by an odd-number of 1/4 of the oscillation period so that the influences of the body motion are imaged at positions where they do not interfere with the original image, that is, at both ends of the field of view. In other words, this system sets the repetition time so that .DELTA..theta.(n) has a suitable form (e.g. Japanese Patent Laid-Open No. 128953/1986).
3-3) The system which makes some contrivances to the pulse sequence so that the speed and acceleration of the sample do not affect the phase component of the NMR signal and thus removes the artifact. In other words, this is the system which keeps always .DELTA..theta.(n)=0 by making some contrivances to the pulse sequence (e.g. "NMR Medical Science", Vol. 7, S.1, p.73 (1987)).
Furthermore, the idea of the fourth stage has been proposed. This is the system which samples only .DELTA..theta.(n) as a separate signal and corrects the signals for the image (e.g. Japanese Patent Laid-Open Nos. 133850/1986 and 20541/1986).
However, the techniques of the first and second stages are not preferable because the specific apparatus fitted to the patient or sample makes the patient uncomfortable. The technique of the fourth stage will be omitted from this discussion because its effect over the technique of the third stage cannot be evaluated quantitatively.
Now, the problems with the technique of the third stage will be discussed in detail. According to the system (3-1), the apparent occurrence of artifact becomes smaller but since .DELTA..theta.(n) is dealt with as noise, its influences are distributed as the noise throughout the image.
The technique (3-2) imposes limitation to the repetition time due to the respiratory period of the patient or sample. The repetition time is one of the important parameters of imaging and any limitation to this factor is not preferable.
Since the technique (3-3) makes the pulse sequence complicated, the minimum echo time that can be attained becomes long. This echo time, too, is another important parameter of imaging and any limitation to it is not preferable, either.